Origins of Li ₂ S‐Rich Layer Formation at Li ₆ PS ₅ Cl|Li Interface: Insights from First‐Principles Simulations

All‐solid‐state lithium batteries (ASSLBs) using sulfide electrolytes such as Li 6 PS 5 Cl (LPSC) offer high ionic conductivity and high safety, yet their performance remains limited by reductive instability at the Li‐metal interface. Here, density functional theory (DFT) calculations combined with ab initio molecular dynamics (AIMD) simulations are used to investigate, with atomistic resolution, the reduction pathways, charge–transfer processes, and structural rearrangements at the thermodynamically preferred LPSC(111)|Li(100) interface. The simulations reveal a strongly localized interfacial reduction process in which PS 4 tetrahedra undergo rapid, layer‐dependent decomposition at the Li‐contacting surface, proceeding through sequential PS bond cleavage to form PS 3 , PS 2 , PS, intermediates, followed by formation of isolated P and S species. Coordination number and radial distribution function (RDF) analyses show that Li 2 S forms primarily from sulfur (S) initially coordinated in Li 6 S units, whereas S released from PS 4 units stabilizes mainly as partially reduced Li x S intermediates. Bader charge analysis further demonstrates that electron transfer is confined within the top Li‐metal layers and the bottom region of the LPSC slab, driving PS bond cleavage and restructuring of the local sulfide environments. Collectively, these results provide an atomistic‐level mechanistic understanding of Li 2 S‐rich interphase formation at LPSC|Li interfaces, offering quantitative guidance for designing electronically blocking interlayers that can mitigate thiophosphate reduction in next‐generation ASSLBs.